ALBERT

Description: The Western Antarctic Peninsula (WAP) is a hotspot for changes caused by humans. The total column ozone is not expected to increase in the next several decades leading to an increased UVB radiation at the earth’s surface and, hence, on polar organisms. Global warming is accelerated at the WAP in comparison with the global mean and in some areas water temperatures have risen by 2°C in the last 50 years. Seaweeds are an essential part of the polar coastal ecosystem showing a high level of endemism. Little is known how these communities react on stressors such as UV radiation, light intensity, increasing temperatures and sedimentation and decreased salinity (from the melt water run-off from the glaciers). A variety of field and laboratory experiments on seaweed communities and young developmental stages (spores) were performed over the last ten years in order to tackle this question. A focus was lying on the interactive effects of various variables such as UV and grazing or temperature, sedimentation and grazing and the effect of light and temperature among others. We also performed long term colonization experiments over 4 years to elucidate the general patterns of polar seaweed succession. Interactive effects occurred for example between grazing and UV radiation in field experiments and between temperature and light intensity in laboratory experiments. Other data are currently analyzed. Our results showed that juvenile stages are especially vulnerable to environmental perturbations and that the degree of susceptibility is species-specific. Shift in community structure as a response to environmental change are likely to occur with yet unknown consequences for the associated heterotrophic organisms.

Description: The Antarctic Peninsula is one of the regions most seriously affected by climate warming. Rising temperatures are considered a stress factor for the rather stenothermal benthic algae inhabiting in this region. Furthermore, it has also been associated to glacier retreat, which causes increased freshwater inflow and sedimentation into the water column, which consist of environmental changes that impose stress conditions for benthic organisms. Besides abiotic factors, grazing is one of the most important drivers of benthic algal communities, particularly at early stages of succession. The objective of the present study was to evaluate experimentally the effects of environmental perturbations ssociated to climate warming in the Antarctic region- such as increased temperature and sedimentation- and grazing, on benthic algal communities at early stages of succession. Artificial substrates (ceramic tiles, 25 cm2) were installed at a subtidal site in Potter Cove (25 de Mayo/King George Island, South Shetland Islands, Antarctica) and, after eleven months of colonization, a trifactorial experiment was performed to test the effects of temperature (2 °C and 6 °C), sediment load (with and without added sediment) and grazing by the gastropod Nacella concinna (ungrazed and grazed) on eveloped communities. Samplings were performed after 12, 14 and 36 days. Species composition and abundance of juveniles (〉1mm) and early stages (〉0.02mm) of macroalgae, the structure of diatom assemblages (taxonomic composition, density and ecological indexes), biomass and photosynthetic efficiency were assessed. Tiles were dominated by juvenile thalli of Palmaria decipiens and early stages of P. decipiens, Monostroma hariotii and Adenocystis utricularis, which are typical macroalgal species in early stages of succession at the Antarctic benthos. Nineteen diatom taxa were identified, mostly pennate and typically benthic forms. Increased temperature did not exert any effects on macroalgal cover nor on diatoms density. However, higher temperatures caused higher diversity and evenness in diatom assemblages probably due to changes in the relative abundance of species that are not sensitive to this factor. Sedimentation exerted positive or neutral effects on macroalgal cover and seems to protect the communities at short term, when early stages are more sensitive to high irradiance conditions. However, it negatively affected diatom evenness. Grazing by N. concinna controls diatom assemblages by reducing their density. These consumers can potentially counteract positive (or negative) sedimentation effects by removing the sediment cover. Over longer time scales, higher temperature and sedimentation may limit the photosynthetic efficiency of marine benthic algal communities in Antarctica. Early stages of succession are subject to complex interactions between abiotic and biotic factors in the Antarctic benthos. In a climate change context, the action of multiple changing factors can introduce alterations in the structure and functioning of the communities of Antarctic benthic algae, which play a central role in this system.